Mass spectrometers



April 25, 1961 R. D. CRAIG 2,981,839

V MASS SPECTROMETERS Filed March 17, 1960 3 Sheets-Sheet 1 06597 DEREK CRAIG 9) April 1961 R. D. CRAIG 2,981,839

MASS SPECTROMETERS Filed March 17, 1960 3 Sheets-Shet 2 3' mam/70R s 06697 mm c/eA/G April 1961 R. D. CRAIG 2,981,839

MASS SPECTROMETERS Filed March 17, 1960 V 3 Sheets-Sheet 3 LI I [=1 m g\ J I \g "1 w m l N I 1 Ii: 9

19019454 7 D5? (WA/G "'pl icationNo.3,426of1960. I Ifa photographic plate is used'todetect the ion spec- 2,981,839 Patented Apr. 25, 19 1 MASS SPECTROMETERS Robert Derek Craig, Altrincham, England, assignor to Associated Electrical Industries Limited, London, England, a British company 1 Filed Mar. 17,1960,'S er.No. 15,719 Claims priority, application Great Britain Mar. 26, 1959 4 Claims. (Cl. 250-419) ,The present invention relates to mass spectrometers and in particular to a mass spectrometer which is so constructed that it is suitable for heating at a high temperature. 1

A mass spectrometer is an apparatus which can be adapted for-analysing the constituent elements of a material. The apparatus comprises an ion source, a mass analyser and a detector. The material to be analysed is ionised in the ion source, and the ions so produced are accelerated and formed intoga well defined beam. This beam is projected into-the mass analyser which is conveniently a region of uniform magnetic field with suitably shaped boundaries. The ions,'6n passing through such a magnetic analyser will be deflected laterally. Since the ions are formed from the constituent elements of the material and these elements may have different pofsitions in the periodic table, the ions willhave'different masses. The amount by which each ion is deflected Will essary to use a plurality of photographic plates. It will therefore be necessary to have means for inserting the plates in turn into the magnetic analyser and withdrawing them after they have been fully exposed, without destroying the vacuum within the magnetic analyser. An apparatus which will perform this operation is described in co-pending US. patent application No. 3,428 of 1960.

dependupon the strength-of the magnetic field and-also on the mass of the ion. Hencefion's of different masses will be deflected different amounts and the ion beam will 7 The mass spectrometer must be' evacuated before the ion beam is produced and when very accurate results are required, or it is known that some constituent elements appear in the materialto be analysed in only'very' small quantities, it is essential that vacuum contamination caused by impurities from the components of the mass spectrometer should be reduced to the lowest possible {level before analysing is commenced. In order to reduce the impurities present, to a very small amount, it is normal practice to heat up the whole ofthefmass spectrometer while it is being evacuatedin'order to vaporise the impurities and removethem invapour form.

Once the components of the mass spectrometer have ;been evacuated andlheated to'fremove the impurities, the

analysis of the material can commence. It may be nec- I essary to analysetwo or more, materials and this will neces'sitate the changing of the material in the ion source. .In order to'jchange the material, it may be necessary to open up the ion source thereby destroyingthe vacuum therein. In order to allow the ion source to be opened "up without destroying .jthe vacuum in the other comporients of the mass spectrometer a valve is provided be tween the ion source andthe other components. Such a f valve 'may be as describedin co-pending US. patent aptrumformed in them'agnetic analyser, it may bejrieces- 'sary tolus e twd or more photographic plates foreach. ma-

mass' sp om Fig. 2.

In this apparatus, which includes a magazine for holding the photographic plates in light tight casettes, the magazine can be sealed off from the main body of the magnetic analyser in order to enable the casettes to be removed and replaced in the main body of the magnetic analyser without destroying the vacuum in this main body. The magazine is evacuated during operation of the mass spectrometer and must therefore be re-evacuatcd after the casettes have been replaced.

The magnetic analyser includes electric coils to pro duce the magnetic field and it is undesirable to heat these coils since the insulation thereon may be harmed or destroyed if its temperature is raised above a certain level. The magnetic analyser must therefore be constructed so as to allow the body of the analyser containing the vacuum to be heated up without simultaneouslyheating the coils. Suitable heaters can be applied to the other components of the mass spectrometer, without harming these components.

The object of the present invention is to provide an improved type ofmass spectrometer which may be heated to a high temperature such as 300 C. or higher while it is being evacuated so as to reduce the impurity contaminationto the lowest possible level.

According to the present invention, a mass spectrometer comprises a chamber, means for evacuating said chamber, a pair of magnetic pole pieces extending inwardly towards one another within said chamber and separated laterally by a smallgap, a magnetic yoke formed from a base member which extends beneath said chamber and two removable side members which extend vertically from said base member one on each side of said chamber and adjacent thereto, energising coils surroundmg each of said side members, means for moving said side members laterally away from the side walls of saidchamber, and removable heating pads located between .satd side members andsaid side walls in order to heat up .said'chamber when said side members. are moved laterally away from said side wall.

The mass spectrometer also comprises an ion source and a magazine for holding photographic plates and the ion source and the magazine are connectedcto the chamber of the magnetic analyser; by members having low thermal conductivity. I In order that the invention may be more readily under- .stood, reference will now be made to the accompanying drawings, in which:

-Fig. 1 is an elevational view of amass spectrometer embodying the features of. the invention,

Fig. 2 is an end view on a larger scale of the magnetic analyser of the mass spectrometer illustrated in Fig. 1-,

part sectioned on the plane II-II of Fig. 3, and

Fig. 3 is a plan view of. the magnetic analyser illustrated in Fig. 2 part sectioned on the plane III-4H of Fig. 4 is a 'cross sectional'view of a modified form of the mass spectrometer.

, With reference to Fig. 1, a mass spectrometer comprises a chamber 1 which may be evacuated by means of a pump' line 2.;;- The chamber is supported 'ontwo supports 4' and streets base member 6. The supportsar'e cross-sectional area.

made in the form of thin walled sections of tubing, so

as to reduce the heat transfer path from the chamber. An ion source 7 is connected to one end of chamber 1 by a cylindrical member 8, which has walls with a small The magazine 9 is connected to the other end of the chamber by;a second cylindrical member 11 the walls of which also have a small crosssectional area. .The magazine 9 is constructed so as to hold a plurality of photographic plates together with apparatus for inserting said plates in turn into the chamber 1, so that they receive the ion spectrum-as already described above, and enable an analysis thereof to be made.

The chamber 1 contains two magnetic pole pieces which extend laterally across the inside of the chamber and are separated by a small gap. The outer magnetic circuit is completed by two side members one of which is denoted by 12, .on which are wound two coils, one of which is denoted by 13, and the base member 6. The side members and the base member and the side walls of the chamber are of soft iron or other suitable magnetic material and form a magnetic circuit with the pole pieces when the coils are energised. This forms'the magnetic field into which the ions frorn'the source 7 are projected so as to form a spectrum which is collected by a photographic plate suitably positioned. A mechanism 14 enables the photographic plate within the chamber 1 to be moved laterally as required so that different parts of the plate may be exposed to the ion spectrum.

In order to operate the mass spectrometer, the material to be analysed is first inserted within the ion source 7. Isolating valves are located between the ion source iently of annular form and may consist of electric heatingelements embedded in suitable insulating-material and covered by a metal sheet. It is necessary'to heat up the chamber 1, without raising the temperature of the insulation of the energising coils to too high a level. Therefore provision is made for moving the side members laterally away from the side walls of the chamber 1, and allowing heating pads ofsuitable construction to be placed between the inner faces of the side members and the outer surfaces of the side walls. This arrangement enables the chamber 1 to be heated without undue heating of the energising coils. The supports 4, 5 do not provide a good thermally conducting path from the chamber 1 to the base member 6, so the chamber 1 may be raised to a high relative temperature. 7

Due to the presenceof the heat chokes? and 11, the ion source 7 and the magazine 9 are not necessarily raised to the same temperature'as the chamber 1. In practice, it is inadvisable to raise the magazine 9 to a very high temperature because the materials used to seal the valve between the cylindrical member 11 and the magazine 9 will not withstand high temperatures. This valve can be of the type described in co-pending U.S.

patent application No. 3,428 of 1960. The chamber 1,

on the other hand, is designed to be heated to a temperature of 300 C. or above.

The mass spectrometeris operated after heating has,

taken place. The ions are produced from the material in the ion source 7 and are projected into the magnetic field in the chamber 1 and form a spectrum as described above. This spectrum is collected on aphotographic plate .which'is inserted into the chamber 1 from the magazine 9. When it is necessary to change. the material .being analysed, the valve between the ion source and the chamber 1 is closed and the ion source is opened so as to enable the material to be changed, without destroying the vacuum in the chamber 1. The ion source will then be closed and re-evacuated by means of the pump 20. It may also be heated again by the surrounding heaters 10 either before or after the new material to be analysed has been inserted.

Similarly, if it is required to replace all the casettes in the magazine 9, the valves between the magazine and the chamber 1 are closed, the magazine is opened and the casettes are replaced without destroying the vacuum in the chamber 1. The magazine 9 will then be closed and re-evacuated :by means of the pump 30, and it may also be heated again by the surrounding heaters 10. It is normally not possible to heat the magazine when the photographic plates are in position but the magazine may be evacuated and heated before the casettes with the plates are inserted into the magazine.

The construction of the chamber 1 and the magnetic circuit will now be described in greater detail with reference to Figs. 2 and 3. The chamber 1 is formed with two side walls 15 and 16 of magnetic material, and a top member 17 and a bottom member 18 and two end pieces 19, 21 of non-magnetic material. One side wall 16 is formed with an annular groove 22 which makes the wall slightly flexible. Two pole pieces 23, 24 extend inwardly from the side walls 15, 16 and are separated by a'spacer 25 so as to form azgap 26. The spacer 25 ensures that the gap 26 remains with a substantially constant Width. The

slightlyfiexible wall 16 ensures that, in spite of differences in the coefficients of expansion of the structural members of the chamber 1, the pole pieces can be kept a portant because during this period the spectrometer is not operating and the width of the gap 26 is not important. The width of the gap26 will, of course, vary as the spacer 25 changes dimension but if this is made of a.

material having a low coeflicierit of expansion, this change will not be important.

The holder 27 for a photographic plate is supported fromthe top member 17 of the chamber 1 and can be .moved laterally within the chamber by means of a mechanism 14 which may be of the type described in co-pending U.S. patent application,No.-3,428 of 1960.

The chamber 1 is supported by the members 4, 5 from the base member 6. The magnetic circuit of the ,magnetic analyser is formed by the base member 6, the

two side members 12, 22 which extend vertically from the base member, two core pieces 29, 31.extending laterally inwardly from the two side members and the two energising coils 13 32 which surround the core pieces. The side members 12, 28 together with the coils and the core pieces .can be moved laterally relative to the chamber 1 and the base plate 6 by means of screwed rods 33 which engage with tapped holes in the sidemernbers 12, 28. The lower-endsof the side members 12, 28 are constrained to move in grooves 35, 36 in the base member 6. These grooves ensure accurate alignment of theside members with the pole pieces 23, 24. A gap can therefore be formed between the side walls of the chamber and the core pieces 29, 31 and, when these gaps are formed, heating pads 37, 38 can be placed on the outside 'of-the side walls 15, 16 and attached thereto.

These heating pads enable the chamber to be heated up without heating the coils above a safe temperature. When thechamberf has been heated up by these'pads,

the pads are removed and the two side members 12,28 are moved laterally inwards so that the faces of the core pieces 29, 31 abut against the outer surfaces of the side walls 15, 16.

The magnetic force exerted on the side members is sufliciently great to ensure that the magnetic circuit is continuous when the side members are in position abutting against the side walls without the necessity of any clamping screws.

In order to ensure that the seals between the ion source and the chamber and the member 11 withstand the heat applied by the heating elements, it is desirable that these seals should be made from a deformable metal strip.

The metal used may be gold. It will be appreciated that the temperature to which the mass spectrometer is raised may be 300 C. or greater and so it is essential that the vacuum-tight seals are very well made.

Fig. 4 illustrates an alternative construction of the chamber 1, and is a cross sectional view of the chamber. The two side walls 41, 42 of the chamber are made very thin and are welded to the top and bottom walls 17, 18. The pole pieces 23, 24 are supported from the top and bottom walls by four suitable supports 43, 44, 45, 46. The inner faces of the pole pieces are spaced apart by' two spacers 47, 48. The heating pads 37, 38 are adapted to be located on the outsides of the side walls ll, 42. Since the side walls are thin they can move so as to account for any relative expansion between the pole pieces and the top and bottom walls of the chamber. The top and bottom walls and the side walls of the chamber are made of a non-magnetic material. Since the side walls are thin they do not greatly affect the magnetic circuit.

There is described above a mass spectrometer which is suitable for heating to a high temperature in order to reduce the impurities therein. The particular construction of the mass spectrometer ensures that the insulation of the energising coils of the magnetic circuit is not harmed by the heating.

What I claimis:

l. A mass spectrometer including a magnetic analyser comprising a chamber, side walls to said chamber, means for evacuating said chamber, a pair of magnetic pole pieces extending inwardly towards one another within said chamber and separated laterally by a small gap, at

. magnetic yoke formed from a base member which exsaid side members have been moved laterally away from said side walls, without appreciably heating said magnetic yoke.

2. A mass spectrometer including a magnetic analyser comprising a chamber, side walls to said chamber, means for evacuating said chamber, a pair of magnetic pole pieces extending inwardly towards one another within said chamber and separated laterally by a small gap, a magnetic yoke formed from a base member which extend beneath said chamber and two movable side members which extend vertically from said base member, one on each side of said chamber and adjacent to said side walls, energising coils surrounding each of said side members, and means for respectively connecting said coils to suitable sources of electrical potential, an ion source, a member having a low thermal conductivity connecting said ion source with said magnetic analyser, and means for evacuating and heating said ion source, and removable heating pads adapted to be located between each of said side members and the associated side wall in order to heat up said chamber when said side members have been moved laterally away from said side walls, without appreciably heating said magnetic yoke.

3. A mass spectrometer including a magnetic analyser comprising a chamber, side walls to said chamber, means for evacuating said chamber, a pair of magnetic pole pieces extending inwardly towards one another within said chamber and separated laterally by a small gap, a magnetic yoke formed from a base member which extends beneath said chamber and two movable side members which extend vertically from said base member, one on each side of said chamber and adjacent to said side walls, encrgising coils surrounding each of said side members, and means for respectively connecting said coils to suitable sources of electrical potential, an ion source, a thin walled cylindrical metal sleeve connecting said ion source with said magnetic analyser, and means for evacuating and heating said ion source, and removable heating pads adapted to be located betweeneach of said side members and the associated side wall in order to heat up said chamber when said side members have been moved laterally away from said side walls, without appreciably heating said magnetic yoke.

4. A mass spectrometer including a magnetic analyser comprising a chamber, side walls to said chamber, means for evacuating said chamber, a pair of magnetic pole pieces extending inwardly towards one another within said chamber and separated laterally by a small gap, a magnetic yoke formed from a base member which extends beneath said chamber and two movable side members which extend vertically from said base member, one on each side chamber and adjacent to said side walls, energising coils surrounding each of said side members, and means for respectively connecting said coils to suitable sources of electrical potential, a magazine for holding photographic plates, means for moving said photographic plates in turn into said magnetic analyser, a member having a low thermal conductivity connecting said magazine with said magnetic analyser, and means for evacuating and heating said magazine, and removable heating pads adapted to be located between each of said side members and the associated side wall in order to heat up said chamber when said side members have been moved laterally away from said side walls, without appreciably heating said magnetic yoke.

No references cited. 

